The present invention relates to method and apparatus for generating a solid-state deep UV laser source. In particular, the present invention relates to generate a solid-state deep UV laser source for customized cornea ablation in photo-refractive surgery.
Topography link and/or wavefront guided custom ablation can potentially improve the outcome of photo-refractive surgery to achieve supernormal visual acuity. In a custom-ablation photo-refractive surgery, a computer of the surgical system reads in the patient""s data from a topography or wavefront device and controls the scan of a surgical laser beam to generate a customized ablation profile. It can thus remove corneal irregularity and correct low and high order of refractive errors. In comparison, conventional photo-refractive surgery removes only low order of refractive errors, such as defocusing and astigmatism. In many circumstances, conventional photo-refractive surgery induces extra amount of high order refractive errors and leads to imperfections such as halo and night vision.
The advantageous custom-ablation procedure requires a fine and precise control of laser energy deposition on the cornea with a fast and accurate compensation of the eye movement. Therefore, it is greatly desirable to have a small ablation beam with stable pulse energy, a scanner with high scanning speed, and an eye-tracking device with fast response.
Currently, focused excimer laser beam scanned by a computer-controlled scanner is the only modality to perform custom-ablation surgery. Due to some intrinsic limitations, however, excimer lasers are far from an ideal laser source for this delicate application.
One limitation of excimer lasers is a large pulse-to-pulse energy fluctuation. A fluctuation of 20% or more is common for excimer refractive lasers. This fluctuation degrades significantly the achievable accuracy of energy deposition on the cornea.
Another limitation of excimer lasers is a low repetition rate of pulse generation. A pulse repetition rate of 100 Hz or lower is typically used for refractive surgery. Higher repetition rate usually leads to bigger pulse-to-pulse fluctuation and degrades laser performance. Because ablation time of the custom ablation surgery is preferable to be similar to that of conventional surgery, this low repetition rate limits the beam spot size to about 1-mm on the cornea and thus limits the fineness of ablation profile.
A further limitation of excimer lasers is its poor beam quality. A typical excimer has a rectangular beam profile, and the intensity distribution varies across the beam and changes with the age of laser optics and discharge electrodes. Usually, the beam collimation is poor and the beam spot size on the scanner is big. The scanner mirror, thus, has to be big. Speed of the scanner is limited by the rotation inertia of the mirror and, consequently, poor beam quality of excimer lasers means a slow scanner. A slow scanner prohibits precise disposition of pulses at high repetition rate and forbids fast response of eye tracking.
The present invention recognizes the special needs for custom ablation in photo-refractive surgery and contemplates a solid-state deep UV laser to overcome the above-identified limitations of excimer lasers. In a preferred embodiment described in this disclosure, a solid state laser is designed to meet the special needs for custom-ablation in photo-refractive surgery.
Accordingly, an objective of the present invention is to provide a new and improved deep UV laser source for customized ablation in photo-refractive surgery.
Another objective of the present invention is to provide a new and improved laser source to enable the implement of fast scanning and fast eye tracking for customized ablation in photo-refractive surgery.
A further objective of the present invention is to provide a new and improved laser source to enable fine and precise control of laser ablation profile for customized ablation in photo-refractive surgery.
Another further objective of the present invention is to provide a new and improved solid-state deep UV laser source with kilohertz pulse rate and nanosecond pulse duration.
In an embodiment of a solid-state deep UV laser source designed for custom ablation in photo-refractive surgery, an apparatus of the present invention comprise:
A diode pumped laser oscillator producing nanosecond pulses at a kilohertz pulsed rate, wherein said oscillator is operated at a wavelength around 800 or 1000 nm and generates a pulsed laser beam close to diffraction limit;
A multiple pass, diode pumped laser amplifier amplifying the nanosecond laser pulses to a mJ level;
A wavelength converter converting the amplified pulses to a wavelength around 200 nm and generating deep UV laser pulses to 100-microWatt level.
In a preferred embodiment, the oscillator is a passively-Q-switched microchip laser manufactured by Nanolase of Meylan, France. The microchip laser is modified to pump with a diode laser at a predetermined pulse rate of about 1000 Hz. This microchip laser can produce sub-nanosecond pulses with pulse energy up to 6 xcexcJ at 1064 nm.
In the preferred embodiment, the multiple passes, diode pumped laser amplifier adapts a configuration tough by Hirlimann et al. in Femtosecond Jet Laser Preamplifier, Optics Communications, Vol. 59, No. 1, PP 52, Aug. 1, 1986. The modified configuration enables smaller angular spread of the multiple passes and thus better energy extraction efficiency from the amplifier.
In this preferred embodiment, the wavelength converter adapts an arrangement depicted by Chen et al. in Recent Developments in Barium Borate, SPIE Proceedings, Vol. 681, No. 12, PP 12-19, 1986. The modified wavelength converter employs different non-linear crystals in different stages of harmonics generation to optimize the beam quality and conversion efficiency.
In this preferred embodiment, the solid-state deep UV laser source is tailored to operate at about 1000 Hz and to have pulse energy of about 0.2 mj at a wavelength of 210 nm. The pulse duration is about 1 nanosecond. This pulse duration is particularly chosen to be short enough to generate deep UV efficiently and to be long enough to avoid expensive mode-locking technology. The spot size of the laser beam is about 0.3 mm on cornea and about 1 mm on the scanner mirror. The pulse to pulse fluctuation of this laser source is smaller than 10%, and the quality of the deep UV beam is near diffraction limit.
Consequently, the tailored solid-state deep UV laser source enables the generation of fine-ablation profile for refractive surgery. With a near diffraction-limited beam quality, this laser source makes it possible to use small scanner mirror for achieving fast scanning and engaging fast eye tracking.
The above and other objectives and advantages of the present invention will become more apparent in the following drawings, detailed description, and claims.